1. Field of the Invention
The present invention relates to metal space trusses of the kind comprising two at least generally horizontally extending and vertically spaced lattices, an upper one and a lower one, which are both composed of two intersecting groups of mutually parallel bars which in the respective lattices are joined to each other in a plurality of nodes, and a plurality of struts, each of which forms an oblique angle with the planes of the two lattices and connects a node in the upper lattice to a node in the lower lattice.
2. Description of the Prior Art
In space trusses of the kind above referred to, which are mainly used for supporting roofs over large halls, the struts commonly form a pyramidal pattern between the two lattices, which in most cases have at least generally square openings, although other variants may also be found. Further, it is customary that one lattice, e.g. the lower one, is slightly smaller than the other in length as well as in width, and in practice, the smallest span of a space truss of the kind here in question is rarely less than about 20 meters, and the vertical distance between the two lattices is usually at least about 1 meter.
The basic problem in constructing such space trusses is to keep the total costs down. This can be achieved only by using the cheapest possible material--under due consideration of the demands for strength, of course--and by at the same time simplifying and also reducing as far as possible the preparatory adaptation of the material as well as the assembly work at the building site.
The original method of erecting a space truss of the kind here in question, which is still applied to a certain extent, is to join together by welding lengths of suitable bars or tubes which in advance have only been cut to size. However, in this case the welding work becomes very extensive and time-consuming, and it requires great skill and care from the welder to make the joints sufficiently strong and reliable. In addition, a multiplicity of fixtures must be used in order to avoid deformations. As a result, the total cost of the completed space truss will be high, above all because of high labour costs.
The most common alternatives to the method just described are based on the use of fairly complex node elements, frequently in combination with specially designed end portions on all the adjoining bars and struts. Thereby a considerable part of the preparatory work may be carried out in a workshop, i.e. under the most favorable conditions, but at the same time the assembly work at the building site commonly becomes so intricate that the same can only be carried out by specially trained workers. Further, in most cases the manufacture of the node elements themselves and sometimes also the preparation of the bar and strut end portions to be connected thereto require qualified and expensive machining operations. Also, almost without exception, the node elements require the use of tubular bars and struts, which contributes to an increased material cost. Hence, even if the use of such prefabricated node elements may result in a certain saving of time at the building site, the total cost of the completed space truss will still be high.